1. Field of the Invention
The present invention relates to a linear light source backlight system of an edge lighting type for use in a planar display device. More particularly, the present invention is suitably applicable, for example, to a back light system provided with a light guide plate that feeds light from a light source to a liquid crystal panel, and to a liquid crystal display device incorporating such a backlight system.
2. Description of the Related Art
In recent years, advancements have actively been made in liquid crystal display devices toward larger screens, higher picture quality, higher resolution, further reduced thickness and weight, lower power consumption, and lower cost. In particular, the performance and structure of backlight systems incorporated in liquid crystal display devices hold the key to improvements in the performance of liquid crystal display devices in terms of efficiency, brightness, power consumption, frame width, and slimness, and thus greatly affect the performance of liquid crystal display devices.
Today, backlight systems for use in liquid crystal display devices are divided into a behind-the-panel type, which has a light source arranged at the back of a panel, and an edge-lighting type, which uses a light guide plate and has a light source arranged close to a side edge of the light guide plate. In liquid crystal display devices in which slimness matters, the edge-lighting type is adopted.
Used as light sources in backlight systems are cold cathode fluorescent tubes and light-emitting diodes (hereinafter also referred to as “LEDs”). Compared with cold cathode fluorescent tubes, light-emitting diodes have the advantages of being less expensive, less power-consuming, and less space-consuming.
A disadvantage with using an LED as a light source in an edge-lighting backlight system and arranging the LED close to a side edge of a light guide plate is that, since an LED is a point light source, the LED tends to produce more noticeable unevenness in brightness and chromaticity near where light enters the light guide plate (at the side edge thereof where the LED is arranged), than does a cold cathode fluorescent tube, which is a linear light source. To prevent such unevenness in brightness and chromaticity near where light enters a light guide plate, as shown in FIG. 6, in conventional liquid crystal display devices, it is customary to arrange a plurality (in the example shown, three) of LEDs 120 along a side edge of a light guide plate 200 and a part of the light guide plate 200 near where light enters it is left as non-display portion 202. That is, the light guide plate 200 is laid over an area larger, toward the LEDs 120, than the display area 201 of the liquid crystal display device, and the part of the light guide plate 200 that lies beyond the display area 201 is left as a non-display portion 202 from which no light comes out. Thus, inconveniently, adopting this structure creates a large dead space, and therefore makes it difficult to reduce the frame width in liquid crystal display devices.
As shown in FIG. 7, one type of backlight system that uses LEDs 120 as a light source as described above has a diffusive sheet 500 and lens sheets 600 (hereinafter also referred to as “BEFs”) arranged on the light-exit surface (front surface) of a light guide plate 200, and has a reflective sheet 400 arranged on the back surface of the light guide plate 200. Here, the lens sheets 600 each have an array of prisms formed on one side, and are arranged on the light-exit surface of the light guide plate 200 with the prism surfaces of the lens sheets 600 facing where the viewer is supposed to be (facing forward). In the example shown, two BEFs 600 are arranged on the front surface of the light guide plate with the prism array of one of the lens sheets 600 perpendicular to that of the other. Inconveniently, adopting this structure requires an increased number of components, and results in, in exchange for a better viewing angle characteristic, low brightness in the direction normal to the light-exit surface as a result of the light from LEDs 120 being emitted through a plurality of members (the light guide plate, the diffusive sheet, and the two BEFs) and thus being more diffused than in an edge-lighting backlight system like the one described below.
One well-known example of using a cold cathode fluorescent tube, which is a linear light source, as a light source in an edge-lighting backlight system is disclosed in JP-A-H11-224516. According to JP-A-H11-224516, a cold cathode fluorescent tube is arranged close to a side edge of a light guide plate, and the light-exit surface (front surface) of the light guide plate, or the opposite surface (back surface) thereof, is formed into a coarse surface; a reflective sheet is arranged on the back surface of the light guide plate, and a lens sheet is arranged, with the prism surface thereof facing down, on the front surface of the light guide plate. Here, arranging the lens sheet with the prism surface thereof facing down means that the lens sheet is arranged such that the side thereof on which it has an array of prisms formed faces the light-exit surface of the light guide plate. Inconveniently, however, as described previously, adopting a cold cathode fluorescent tube, compared with an LED, has the disadvantages of being more expensive, more power-consuming, and more space-consuming.
An LED may be used as a light source in the backlight system disclosed in JP-A-H11-224516. Even doing so, however, does not make it any easier to reduce the frame width in liquid crystal display devices because of unevenness in brightness and chromaticity near where light enters the light guide plate.